The only time they have the obligation to decide what is best is if the client is a risk to themselves or others.

Otherwise, you are paying for the therapist's professional opinion, so they get to offer it. They can say that they might know what is best, but it's up to the client to decide if that is true or not, and up to the client to decide whether or not to follow the advice.

My therapist has never told me that she knows best. I've asked her for her opinion about things, and asked her what she thought the best course of action might be, but she's never acted like she has a "right to decide what's best."